Ferritic stainless steel sheet having superior punching workability and method for manufacturing the same

ABSTRACT

A ferritic stainless steel sheet and a method for manufacturing the ferritic stainless steel sheet include a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities is hot-rolled at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C., softening annealing is performed on an obtained hot-rolled steel sheet, picking is further performed, and cold rolling is subsequently performed.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/061498, with an international filing date of Jun. 18, 2008 (WO 2009/025125 A1, published Feb. 26, 2009), which is based on Japanese Patent Application No. 2007-213801, filed Aug. 20, 2007, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a ferritic stainless steel sheet having a superior punching workability and a method for manufacturing the same.

BACKGROUND

Since having a superior corrosion resistance and being easily worked, a ferritic stainless steel sheet has been used in various applications, such as architectural materials, transport machines, electric home appliances, and kitchen instruments. To manufacture these structures, after a ferritic stainless steel sheet is cut into a predetermined shape, a work, such as forming or welding, is further performed. For cutting of a ferritic stainless steel sheet, a shearing work, which has a high productivity, has been widely used.

In a shearing work, burrs are generated at a cross section of a ferritic stainless steel sheet. In the case in which the height of burrs is large, (a) when a ferritic stainless steel sheet which is cut out is transported to a forming machine (such as a press forming machine), trouble may arise due to the presence of the burrs, and

(b) when welding is performed, since a space may be generated at a burr position of a ferritic stainless steel sheet which is to be welded, for example, burn through may disadvantageously occur. Burrs are not only generated by a shearing work but are also generated by a punching work as shown in FIG. 1B. Hence, development of a punching technique and/or a shearing technique that generates no burrs has been desired.

In the punching work, since a cutting plane is also formed by shearing, the punching work and the shearing work are essentially the same. That is, a generation mechanism of burrs by the punching work is the same as that by the shearing work.

However, heretofore, an investigation to prevent the generation of burrs caused by the punching work and/or the shearing work has not been sufficiently performed, and an investigation to suppress the generation of burrs through an improvement in formability of a steel sheet has been performed.

For example, in Japanese Unexamined Patent Application Publication No. 10-204588, a technique has been disclosed in which recrystallization is facilitated by defining components of a hot-rolled steel sheet and a coiling temperature thereof. According to that technique, to improve the formability, the contents of C, P, and S are decreased to decrease precipitates of FeTiP, Ti₄C₂S₂, TiC, and the like. However, in the punching work and/or the shearing work, a large burr is generated.

In Japanese Unexamined Patent Application Publication No. 2002-249857, a technique has been disclosed in which crystal grains of ferrite are coarsened by defining components of a ferritic stainless steel sheet. In that technique, crystal grains of ferrite are coarsened (GSN 5.5 to 8.0) to improve stretch formability of a ferritic stainless steel sheet. However, in the punching work and/or the shearing work, a large burr is liable to be generated.

In Japanese Unexamined Patent Application Publication No. 2002-12955, a technique for improving press formability has been disclosed in which Ti is added, and TiO₂ and Al₂O₃ are suppressed from being precipitated. However, even when the ferritic stainless steel sheet according to this technique is used, in the punching work and/or the shearing work, a large burr is also liable to be generated.

It could therefore be helpful to provide a ferritic stainless steel sheet which can be processed by a punching work and/or a shearing work without generating burrs and a method for manufacturing the above ferritic stainless steel sheet. Hereinafter, the punching work and the shearing work are collectively called “punching work.”

SUMMARY

We researched causes of burrs generated when punching work is performed on a ferritic stainless steel sheet. As a result, we found the following:

-   -   (A) When a NbTi complex carbonitride is precipitated in grain         boundaries of ferrite crystal grains of a ferritic stainless         steel sheet, cracks caused by a punching work are likely to be         propagated, and as a result, the generation of burrs can be         prevented.     -   (B) When the average ferrite crystal grain size of a ferritic         stainless steel sheet measured in accordance with ASTM E 112 is         set to 20 μm or less, a NbTi complex carbonitride can be         uniformly dispersed.     -   (C) When the yield ratio of a ferritic stainless steel sheet is         set to 0.65 or more, work hardening caused by a punching work is         suppressed, and propagation of cracks is facilitated, so that         the generation of burrs can be prevented. This disclosure is         based on the above findings.

That is, we provide a ferritic stainless steel sheet having a superior punching work-ability, which comprises: a composition containing 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities; and a structure in which an average ferrite crystal grain size is 20 μm or less, and a ratio [Nb]/[Ti] of a Nb content to a Ti content contained in a NbTi complex carbonitride present in ferrite crystal grain boundaries is in the range of 1 to 10. In addition, the ferrite crystal grain size is an ASTM nominal grain diameter obtained in accordance with ASTM E 112.

In addition, according to the above ferritic stainless steel sheet, the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.

In addition, the ferritic stainless steel sheet further comprises 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

In addition, we provide a method for manufacturing a ferritic stainless steel sheet having a superior punching workability, which comprises: performing hot-rolling of a slab having a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C.; performing softening annealing of an obtained hot-rolled steel sheet; then performing pickling; then performing cold rolling; and performing recrystallization annealing of an obtained cold-rolled steel sheet.

In addition, according to the above method for manufacturing a ferritic stainless steel sheet, the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.

In addition, the method for manufacturing a ferritic stainless steel sheet further comprises 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.

In addition, according to the method for manufacturing a ferritic stainless steel sheet, a slab heating temperature is 1,000° C. or less.

A ferritic stainless steel sheet can be manufactured which can be processed by a punching work without generating large burrs which cause an industrial problem.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show side views of a punching device before and after, respectively, a punching work is performed.

FIG. 2 shows a plan view and a side view of a punched-out hole formed by punching out a disc having a diameter of 10 mm.

DETAILED DESCRIPTION

First, the reasons for selecting components of a ferritic stainless steel sheet will be described. In addition, as described above, a punching work and a shearing work are collectively called a punching work.

C: 0.0030 to 0.012 Mass Percent

C is an element that binds to Cr, which will be described later, to form a Cr carbide which makes stainless steel sensitive to corrosion. Hence, by addition of Ti and Nb, C is fixed in the form of a NbTi complex carbonitride, and the NbTi complex carbonitride is dispersed and precipitated, so that the generation of burrs caused by a punching work is prevented. When the C content is less than 0.0030 mass percent, the above effect cannot be obtained. On the other hand, when the content is more than 0.012 mass percent, the generation of a Cr carbide cannot be suppressed, and the corrosion resistance is degraded. In addition, since the amount of the NbTi complex carbonitride is increased, and ferrite grains are liable to be expanded and coarsened, burrs are liable to be generated. Hence, the C content is set in the range of 0.0030 to 0.012 mass percent. More preferably, the content is 0.004 to 0.010 mass percent.

Si: 0.13 Mass Percent Or Less

Si is an element that hardens a ferritic stainless steel sheet by solid-solution hardening to degrade the ductility. When the Si content is more than 0.13 mass percent, the ductility of a ferritic stainless steel sheet is considerably degraded. Hence, the Si content is set to 0.13 mass percent or less. More preferably, the content is 0.10 mass percent or less.

Mn: 0.25 Mass Percent Or Less

Mn is an element that degrades the corrosion resistance of a ferritic stainless steel sheet. When the Mn content is more than 0.25 mass percent, in addition to the degradation in corrosion resistance, Mn binds to S which will be described later, and as a result, fine MnS is liable to be generated. MnS is precipitated in grain boundaries of ferrite crystal grains and expands the ferrite crystal grains by hot rolling and/or cold rolling, and as a result, burrs having a large height are generated in a punching work. Hence, the Mn content is set to 0.25 mass percent or less. More preferably, the content is 0.20 mass percent or less.

P: 0.04 Mass Percent Or Less

P is an element that hardens a ferritic stainless steel sheet by solid-solution hardening to degrade the toughness. When the P content is more than 0.04 mass percent, the toughness of a ferritic stainless steel sheet is considerably degraded. Hence, the P content is set to 0.04 mass percent or less. More preferably, the content is 0.03 mass percent or less.

S: 0.005 Mass Percent Or Less

S is an element that binds to Mn or Ti, which will be described later, to form MnS or TiS and disturb equiaxial crystallization of ferrite crystal grains. When the S content is more than 0.005 mass percent, since ferrite crystal grains are considerably expanded, burrs having a large height are generated in a punching work. Hence, the S content is set to 0.005 mass percent or less. More preferably, the content is 0.003 mass percent or less.

Al: 0.06 Mass Percent Or Less

Al is used as a deoxidizing agent in a steelmaking process for forming ferritic stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. When the Al content is more than 0.06 mass percent, Al binds to N, and AlN is liable to be generated. AlN expands ferrite crystal grains by hot rolling and/or cold rolling, so that burrs having a large height are generated in a punching work. Hence, the Al content is set to 0.06 mass percent or less. However, when the Al content is less than 0.02 mass percent, the deoxidizing effect cannot be obtained in a steelmaking process. Accordingly, the Al content is preferably in the range of 0.02 to 0.06 mass percent. More preferably, the content is 0.02 to 0.045 mass percent.

N: 0.0030 to 0.012 Mass Percent

N generates a NbTi complex carbonitride. When a NbTi complex carbonitride is uniformly dispersed in a ferritic stainless steel sheet, cracks generated by a punching work are likely to be propagated, so that the generation of burrs can be prevented. When the N content is less than 0.0030 mass percent, a sufficient NbTi complex carbonitride amount is not generated. On the other hand, when the content is more than 0.012 mass percent, a Cr nitride is precipitated, so that the corrosion resistance is degraded. Hence, the N content is set in the range of 0.0030 to 0.012 mass percent. More preferably, the content is 0.0040 to 0.010 mass percent.

Cr: 20.5 to 23.5 Mass Percent

Cr is an element for forming a passivation film on a surface of a ferritic stainless steel sheet to improve the corrosion resistance. When the Cr content is less than 20.5 mass percent, a superior corrosion resistance to that of a stainless steel containing 18% of Cr cannot be obtained. On the other hand, when the content is more than 23.5%, since a hard phase containing Cr and Nb is liable to be precipitated, the workability is degraded, and in addition, recrystallization by annealing after hot rolling (hereinafter referred to as “soft annealing”) and/or by annealing after cold rolling (hereinafter referred to as “recrystallization annealing”) is disturbed, so that ferrite crystal grains are liable to be expanded in a rolling direction. When the ferrite crystal grains are expanded, burrs having a large height are liable to be generated in a punching work. Hence, the Cr content is set in the range of 20.5 to 23.5 mass percent. More preferably, the content is 20.5 to 22.5 mass percent.

Cu: 0.3 to 0.6 Mass Percent

Cu has a function to further improve the corrosion resistance of a ferritic stainless steel sheet containing 20.5 mass percent or more of Cr. When the Cu content is less than 0.3 mass percent, the above effect cannot be obtained. On the other hand, when the content is more than 0.6 mass percent, Cu binds to S, and hence CuS is liable to be generated. CuS expands ferrite crystal grains by hot rolling and/or cold rolling and generates burrs having a large height in a punching work. Hence, the Cu content is set in the range of 0.3 to 0.6 mass percent. More preferably, the content is 0.3 to 0.5 mass percent. Even more preferably, the content is 0.3 to 0.45 mass percent.

Ni: 0.5 Mass Percent Or Less

Ni has a function to further improve the corrosion resistance of a ferritic stainless steel sheet. To obtain the above effect, the content is preferably 0.1 mass percent or more. However, when the Ni content is more than 0.5 mass percent, a ferritic stainless steel sheet is hardened, and as a result, the ductility thereof is degraded. Hence, the Ni content is set to 0.5 mass percent or less. More preferably, the content is 0.4 mass percent or less.

Nb: 0.3 to 0.5 Mass Percent

Nb has a function to generate a NbTi complex carbonitride in a ferritic stainless steel sheet and to facilitate propagation of cracks generated in a punching work, so that the generation of burrs can be prevented. When the Nb content is less than 0.3 mass percent, a large amount of Cr carbonitride is precipitated, and as a result, the corrosion resistance of a ferritic stainless steel sheet is degraded. On the other hand, when the content is more than 0.5 mass percent, a hard phase containing Cr and Nb is generated, the workability is degraded, and in addition, since the NbTi complex carbonitride is not likely to be generated, burrs having a large height are generated in a punching work. Hence, the Nb content is set in the range of 0.3 to 0.5 mass percent. More preferably, the content is 0.3 to 0.45 mass percent.

Ti: 0.05 to 0.15 mass percent

Ti has a function to generate a NbTi complex carbonitride in a ferritic stainless steel sheet and to facilitate propagation of cracks generated in a punching work, so that the generation of burrs can be prevented. When the Ti content is less than 0.05 mass percent, the NbTi complex carbonitride is not generated, and a Ti carbonitride and/or a Nb carbonitride is precipitated in ferrite crystal grains. As a result, burrs having a large height are generated in a punching work. On the other hand, when the content is more than 0.15 mass percent, a large amount of TiS is precipitated, equiaxial crystallization of ferrite grains is disturbed, and as a result, burrs having a large height are generated in a punching work. Hence, the Ti content is set in the range of 0.05 to 0.15 mass percent. More preferably, the content is 0.05 to 0.12 mass percent.

The balance other than the components described above contains Fe and inevitable impurities. The amount of the inevitable impurities is preferably decreased as small as possible.

Furthermore, in the ferritic stainless steel sheet, at least one selected from the group consisting of B, Mo, V, and Ca is preferably contained.

For example, 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca may be contained. B: 0.001 mass percent or less

When a very small amount of B is added, recrystallization nuclei are formed, and as a result, an effect of grain refining crystal grains is obtained. To obtain the above effect, the content is preferably 0.0001 mass percent or more. However, when more than 0.001 mass percent of B is added, the workability may be degraded due to steel hardening, and surface defects may occur. Hence, the B content is set to 0.001 mass percent or less.

Mo: 0.1 Mass Percent Or Less

Mo is an element to strengthen a passivation film, facilitate re-passivation after corrosion generation, and improve the corrosion resistance of stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. However, when more than 0.1 mass percent is added, the workability, such as press workability, is degraded by solid solution strengthening. Hence, the Mo content is set to 0.1 mass percent or less.

V: 0.05 Mass Percent Or Less

V is an element to improve the corrosion resistance of stainless steel. To obtain the above effect, the content is preferably 0.01 mass percent or more. However, when more than 0.05 mass percent is added, steel is hardened, and as a result, the workability is degraded. Hence, the V content is set to 0.05 mass percent or less.

Ca: 0.01 Mass Percent Or Less

Ca is an element to prevent molten steel from adhering to steelmaking devices, such as a nozzle. This effect can be obtained at a content of 0.001 mass percent or more. However, when more than 0.01 mass percent is added, Ca is precipitated in the form, for example, of CaO and CaS in steel. Since these inclusions are easily dissolved in water and increase a local pH, corrosion starts therefrom. Hence, the Ca content is set to 0.01 mass percent or less.

Next, a structure of the ferritic stainless steel sheet will be described. Average grains size of ferrite crystal grains: 20 μm or less

The size of ferrite crystal grains of a ferritic stainless steel sheet has a significant influence on the height of burrs generated by a punching work. When the grain size is more than 20 μm, deformation of each ferrite crystal grain is increased, and hence, burrs having a large height are liable to be generated. Accordingly, the grain size of ferrite crystal grains is set to 20 μm or less. Incidentally, the ferrite crystal grain size is an ASTM nominal grain diameter obtained in accordance with ASTM E 112.

Ratio [Nb]/[Ti] Between Nb Content And Ti Content Contained In Nbti Complex Carbonitride: 1 to 10

Cracks caused by a punching work are generated from interfaces between ferrite crystal grains and precipitates present in grain boundaries thereof and are propagated along the grain boundaries. Hence, when a NbTi complex carbonitride is made to be precipitated in grain boundaries of ferrite crystal grains, and when a great number of cracks are made to be generated from the carbonitride and are further made to be combined with each other, cutting can be easily performed. As a result, the generation of burrs in a punching work can be prevented. When the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is less than 1, adhesion between ferrite grain boundaries and the NbTi complex carbonitride is increased in a punching work, cracks are not likely to be generated, and as a result, the height of burrs is increased. On the other hand, when the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is more than 10, the NbTi complex carbonitride is particularized, and as a result, cracks are also not likely to be generated at interfaces formed with ferrite grain boundaries. Hence, the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride is set in the range of 1 to 10.

In addition, as a method for measuring the ratio [Nb]/[Ti] between the Nb content and the Ti content contained in the NbTi complex carbonitride, after a thin film is formed from a central portion of a ferritic stainless steel sheet in a thickness direction by a twin jet method, the Nb content [Nb] and the Ti content [Ti] of the NbTi complex carbonitride (inclusions in which a Nb carbonitride and a Ti carbonitride are mixed together on an atomic level, or precipitated inclusions in which one carbonitride functions as precipitation sites and the other carbonitride adheres thereto) precipitated in grain boundaries are measured by a transmission electron microscope, and the [Nb]/[Ti] value is calculated.

Next, mechanical properties of the ferritic stainless steel sheet will be described.

Yield ratio: 0.65 Or More

When the yield ratio of a ferritic stainless steel sheet is less than 0.65, since work hardening is liable to occur by a punching work, deformation of each ferrite crystal grain is increased, and hence burrs having a large height are liable to be generated in a punching work. The ferritic stainless steel sheet has a yield ratio of 0.65 or more.

Next, a method for manufacturing the ferritic stainless steel sheet will be described.

After ferritic stainless steel having predetermined components is formed by melting and is then further formed into a slab, hot rolling (finishing temperature: 900° C. or more, coiling temperature: 400 to 550° C.) is performed by heating to 1,000° C. or more, so that a hot-rolled steel sheet is formed.

Heating Temperature Of Slab: 1,000° C. Or More

Carbides and nitrides are once melted by heating a slab, and the finishing temperature and the coiling temperature are defined, so that a NbTi complex carbonitride is made to be precipitated in grain boundaries of ferrite crystal grains. Hence, the heating temperature of the slab is preferably set to 1,000° C. or more. In this case, since the slab is deformed at a high temperature, and manufacturing cannot be easily performed, the upper limit of the slab heating temperature is 1,250° C. A more preferable range is 1,050 to 1,200° C.

Finishing Temperature: 900° C. Or More

When the finishing temperature is less than 900° C., recrystallization is disturbed during hot rolling, so that ferrite crystal grains are expanded in a rolling direction by hot rolling. Hence, burrs having a large height are liable to be generated when a ferritic stainless steel sheet is processed by a punching work. Accordingly, the finishing temperature is set to 900° C. or more. In addition, by the reason to prevent seizing with a rolling roll, the upper limit of the finishing temperature is 1,050° C. More preferably, the finishing temperature is in the range of 920 to 1,000° C.

Coiling Temperature: 400 to 550° C.

The coiling temperature of a hot-rolled steel sheet has an important function to precipitate a NbTi complex carbonitride in grain boundaries of ferrite crystal grains. When the coiling temperature is less than 400° C., the NbTi complex carbonitride is not precipitated. More preferably, the coiling temperature is in the range of 450 to 530° C.

On the other hand, when the coiling temperature of a hot-rolled steel sheet is more than 550° C., a hard phase containing Nb and Cr is precipitated, and as a result, the toughness is considerably degraded.

Hence, the coiling temperature of a hot-rolled steel sheet is set in the range of 400 to 550° C. When the coiling temperature is in this range, the NbTi complex carbonitride is precipitated in grain boundaries of ferrite crystal grains.

The hot-rolled steel sheet thus obtained is processed by softening annealing and is further processed by pickling. Conditions of the softening annealing and those of the pickling are not particularly limited, and these processes are performed in accordance with known methods. For example, as a preferable condition range for the softening annealing, the temperature is 900 to 1,100° C., and the time is 30 to 180 seconds. Next, cold rolling is performed, so that a cold-rolled steel sheet is obtained. The cold-rolled steel sheet thus obtained is processed by recrystallization annealing, so that a ferritic stainless steel sheet is obtained. Conditions of the cold rolling and those of the recrystallization annealing are not particularly limited, and these processes are performed in accordance with known methods. For example, as a preferable condition range for the recrystallization annealing, the temperature is 900 to 1,100° C., and the time is 30 to 180 seconds. In addition, the cold-rolled steel sheet may be processed by temper rolling. The draft of the temper rolling is preferably in the range of 0.5% to 1.5%.

EXAMPLES

After each ferritic stainless steel having components shown in Table 1 was formed by melting and was further molded into a slab, hot rolling was performed, so that a hot-rolled steel sheet having a thickness of 3 mm was obtained. The conditions of the hot rolling were shown in Table 2. The hot-rolled steel sheet thus obtained was processed by softening annealing (temperature: 900 to 1,100° C., time: 100 to 500 seconds) and was further processed by pickling. Subsequently, cold rolling was performed, so that a cold-rolled steel sheet having a thickness of 0.8 mm was obtained.

The cold-rolled steel sheet thus obtained was processed by recrystallization annealing (temperature: 900 to 1,100° C., time: 100 to 500 seconds) and was further processed by pickling.

After a thin film was formed from a central portion of the ferritic stainless steel sheet thus formed in a thickness direction by a twin jet method, a Nb content [Nb] and a Ti content [Ti] of a NbTi complex carbonitride precipitated in grain boundaries were measured by a transmission electron microscope, and the [Nb]/[Ti] value was calculated. After a structure was exposed by polishing a sheet-thickness cross section parallel to the rolling direction, the ferrite grain size was observed using an optical microscope. Next, 5 line segments each having an actual length of 500 μm were drawn on a photograph in each of a longitudinal direction and a lateral direction, and the number of intersections between the line segments and crystal grain boundaries shown in the photograph was counted. The ASTM nominal grain diameter was obtained in such a way that the total length of the line segments was divided by the number of intersections, and the value obtained thereby was multiplied by 1.13. The results are shown in Table 2. In addition, the measurement of the grain size was performed using one arbitrary viewing field.

In addition, a JIS-No. 13B tensile test piece was formed from the ferritic stainless steel sheet, and a tensile test was performed. The results are shown in Table 2. The tensile test piece was obtained so that a tensile direction was parallel to the rolling direction.

Furthermore, a punching test piece (100 mm by 100 mm) was obtained by cutting the ferritic stainless steel sheet, and a punching test was performed using a punching device shown in FIGS. 1A and 1B. After a round hole having a diameter of 10 mm was formed by a punching work at a central portion of the punching test piece, the height of burrs was measured. The results are shown in Table 2. In addition, in FIG. 2, a schematic view of a burr of a punched-out hole formed by punching out a disc having a diameter of 10 mm is shown. The height of the burr of one round hole was measured at 4 points at 90° regular intervals, and the average of the height was obtained therefrom.

Nos. 1 to 5 of Table 2 are examples in each of which the C content was changed. Although the height of the burr of Nos. 2 to 4 which were within our range was 50 μm or less, in Nos. 1 and 5 which were out of our range, a burr having a height of more than 100 μm was generated.

Nos. 6 to 10 are examples in each of which the Nb content was changed. The height of the burr of Nos. 7 to 9 which were within our range was 50 μm or less. In No. 6 in which the Nb content was lower than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 10 in which the Nb content was higher than our range, the ferrite crystal grains were expanded, and a burr having a height of more than 100 μm was generated.

Nos. 11 to 15 are examples in each of which the Ti content was changed. The height of the burr of Nos. 12 to 14 which were within our range was 50 μm or less. In No. 11 in which the Ti content was lower than our range, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Since the amount of precipitation of the NbTi complex carbonitride was small, a burr having a height of more than 100 μm was generated. In No. 15 in which the Ti content was higher than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.

Nos. 16 to 20 are examples in each of which the N content was changed. The height of the burr of Nos. 17 to 19 which were within our range was 50 μm or less. In No. 16 in which the N content was lower than our range, since the amount of the NbTi complex carbonitride was small, and the [Nb]/[Ti] value was small, a burr having a height of more than 100 μm was generated. In No. 20 in which the N content was higher than our range, in addition to a high [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.

Nos. 21 to 25 are examples in which the conditions of the hot rolling were changed. The height of the burr of Nos. 23 and 24 which were within our range was 50 μm or less. In No. 21 in which the finishing temperature and the coiling temperature were out of our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 22 in which the coiling temperature was lower than our range, in addition to a low [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated. In No. 25 in which the coiling temperature was higher than our range, in addition to a high [Nb]/[Ti] value, the grain size of the ferrite crystal grains was large, and the yield ratio was small. Hence, a burr having a height of more than 100 μm was generated.

TABLE 1 COMPONENT (MASS PERCENT) No. C Si Mn P S Cr Al N Cu Ni Nb Ti 1 0.0011 0.13 0.16 0.028 0.002 20.8 0.041 0.0077 0.39 0.13 0.38 0.08 2 0.0048 0.13 0.16 0.028 0.002 20.8 0.041 0.0077 0.39 0.13 0.38 0.08 3 0.0085 0.13 0.16 0.028 0.003 20.8 0.041 0.0077 0.39 0.12 0.38 0.08 4 0.0105 0.13 0.16 0.028 0.002 20.8 0.041 0.0077 0.39 0.14 0.38 0.08 5 0.0023 0.13 0.16 0.028 0.002 20.8 0.041 0.0077 0.39 0.12 0.38 0.08 6 0.0066 0.06 0.21 0.032 0.001 22.3 0.024 0.0098 0.56 0.25 0.12 0.12 7 0.007 0.06 0.21 0.033 0.001 22.3 0.025 0.0098 0.56 0.25 0.35 0.12 8 0.0068 0.06 0.21 0.031 0.001 22.3 0.024 0.0098 0.56 0.25 0.43 0.12 9 0.0066 0.06 0.21 0.032 0.001 22.3 0.025 0.0098 0.56 0.25 0.48 0.12 10 0.0066 0.06 0.21 0.032 0.001 22.3 0.025 0.0098 0.56 0.24 0.65 0.12 11 0.0102 0.08 0.13 0.018 0.004 20.5 0.055 0.0065 0.43 0.42 0.41 0.001 12 0.0107 0.08 0.13 0.018 0.004 20.5 0.055 0.0065 0.43 0.42 0.41 0.07 13 0.0105 0.08 0.13 0.018 0.004 20.5 0.055 0.0065 0.43 0.42 0.41 0.11 14 0.0108 0.08 0.13 0.018 0.004 20.5 0.055 0.0065 0.43 0.42 0.41 0.14 15 0.0105 0.08 0.13 0.018 0.004 20.5 0.055 0.0065 0.43 0.42 0.41 0.26 16 0.0057 0.05 0.18 0.036 0.001 21.2 0.041 0.0011 0.41 0.33 0.48 0.07 17 0.0059 0.05 0.18 0.035 0.001 21.2 0.041 0.0039 0.41 0.33 0.48 0.07 18 0.0055 0.05 0.18 0.035 0.001 21.2 0.041 0.0066 0.41 0.33 0.48 0.07 19 0.0057 0.05 0.18 0.036 0.001 21.2 0.041 0.0105 0.41 0.33 0.48 0.07 20 0.0023 0.08 0.18 0.035 0.001 22.0 0.041 0.0212 0.41 0.33 0.48 0.07 21 0.0082 0.12 0.16 0.036 0.002 23.0 0.034 0.0107 0.34 0.44 0.34 0.09 22 0.0082 0.12 0.16 0.036 0.002 23.0 0.034 0.0102 0.34 0.44 0.34 0.09 23 0.0085 0.12 0.16 0.036 0.002 23.0 0.034 0.0105 0.34 0.44 0.34 0.09 24 0.0083 0.12 0.16 0.036 0.002 23.0 0.034 0.0107 0.34 0.44 0.34 0.09 25 0.0083 0.12 0.16 0.036 0.002 23.0 0.034 0.0108 0.34 0.45 0.34 0.09

TABLE 2 HOT ROLLING FERRITE HEAT- FINISH- CRYSTAL PUNCHING ING ING COILING NbTi GRAINS MECHANICAL PROPERTIES WORK TEMPER- TEMPER- TEMPER- COMPOSITE GRAIN TENSILE ELON- BURR ATURE ATURE ATURE CARBONITRIDE SIZE YIELD STRENGTH GATION HEIGHT No. ° C.) (° C.) (° C.) [Nb]/[Ti] (μm) RATIO (MPa) (%) (μm) REMARKS 1 1200 940 420 4.4 32 0.61 440 36 123 COMPARATIVE EXAMPLE 2 1200 940 420 4.4 18 0.74 465 35 30 INVENTION 3 1200 940 420 4.4 15 0.75 476 34 31 EXAMPLE 4 1200 940 420 4.4 15 0.76 480 32 33 5 1200 940 420 4.4 26 0.80 539 26 135 COMPARATIVE (EXPANDED EXAMPLE GRAIN) 6 1170 980 400 0.9 25 0.62 451 34 110 7 1170 980 400 2.7 16 0.75 468 34 36 INVENTION 8 1170 980 400 3.3 17 0.76 477 33 32 EXAMPLE 9 1170 980 400 3.7 17 0.76 484 33 33 10 1170 980 400 5.0 39 0.61 446 35 132 COMPARATIVE (EXPANDED EXAMPLE GRAIN) 11 1150 900 400 — 26 0.63 451 34 105 12 1150 900 400 5.4 18 0.76 467 33 33 INVENTION 13 1150 900 400 3.4 17 0.77 473 33 34 EXAMPLE 14 1150 900 400 2.7 16 0.77 479 32 35 15 1150 900 400 0.8 28 0.61 443 33 122 COMPARATIVE (EXPANDED EXAMPLE GRAIN) 16 1180 950 440 0.6 17 0.75 465 33 131 17 1180 950 440 6.3 16 0.74 466 32 45 INVENTION 18 1180 950 440 6.8 15 0.75 467 32 37 EXAMPLE 19 1180 950 440 7.1 15 0.76 465 33 34 20 1180 950 440 1.6 26 0.63 466 35 139 COMPARATIVE 21 1160 800 400 0.6 45 0.64 478 32 144 EXAMPLE (EXPANDED GRAIN) 22 1160 950 350 0.7 36 0.66 468 34 137 (EXPANDED GRAIN) 23 1160 950 440 3.5 16 0.73 478 33 42 INVENTION 24 1160 950 500 3.5 17 0.72 481 32 43 EXAMPLE 25 1160 950 650 2.3 25 0.65 467 31 146 COMPARATIVE (EXPANDED EXAMPLE GRAIN) 

1. A ferritic stainless steel sheet comprising: a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities; and a structure in which a ferrite crystal grain size is 20 μm or less, and a ratio [Nb]/[Ti] of a Nb content to a Ti content contained in a NbTi complex carbonitride present in ferrite crystal grain boundaries is in the range of 1 to
 10. 2. The ferritic stainless steel sheet according to claim 1, wherein the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.
 3. The ferritic stainless steel sheet according to claim 1, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
 4. A method for manufacturing a ferritic stainless steel sheet comprising: performing hot-rolling of a slab having a composition which contains 0.0030 to 0.012 mass percent of C, 0.13 mass percent or less of Si, 0.25 mass percent or less of Mn, 0.04 mass percent or less of P, 0.005 mass percent or less of S, 0.06 mass percent or less of Al, 0.0030 to 0.012 mass percent of N, 20.5 to 23.5 mass percent of Cr, 0.3 to 0.6 mass percent of Cu, 0.5 mass percent or less of Ni, 0.3 to 0.5 mass percent of Nb, 0.05 to 0.15 mass percent of Ti, and the balance being Fe and inevitable impurities at a finishing temperature of 900° C. or more and at a coiling temperature of 400 to 550° C.; performing softening annealing of an obtained hot-rolled steel sheet; performing pickling; performing cold rolling; and performing recrystallization annealing of an obtained cold-rolled steel sheet.
 5. The method for manufacturing the ferritic stainless steel sheet according to claim 4, wherein the Nb content is 0.3 to 0.45 mass percent, and the Ti content is 0.05 to 0.12 mass percent.
 6. The method for manufacturing the ferritic stainless steel sheet according to claim 4, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
 7. The method for manufacturing a ferritic stainless steel sheet according to one of claim 4, wherein a slab heating temperature is 1,000° C. or more.
 8. The ferritic stainless steel sheet according to claim 2, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
 9. The method for manufacturing the ferritic stainless steel sheet according to claim 5, further comprising 0.001 mass percent or less of B, 0.1 mass percent or less of Mo, 0.05 mass percent or less of V, and 0.01 mass percent or less of Ca.
 10. The method for manufacturing a ferritic stainless steel sheet according to one of claim 5, wherein a slab heating temperature is 1,000° C. or more.
 11. The method for manufacturing a ferritic stainless steel sheet according to one of claim 6, wherein a slab heating temperature is 1,000° C. or more.
 12. The method for manufacturing a ferritic stainless steel sheet according to one of claim 9, wherein a slab heating temperature is 1,000° C. or more. 